High-strength hot-rolled steel sheet and manufacturing method therefor

Abstract

Provided is a high-strength hot-rolled steel sheet that exhibits little inter-coil variability in mechanical properties and excellent stretch flangeability. By mass, said steel sheet contains more than 0.010% and no more than 0.06% carbon, up to 0.3% silicon, up to 0.8% manganese, up to 0.03% phosphorus, up to 0.02% sulfur, up to 0.1% aluminum, up to 0.01% nitrogen, and 0.05-0.10% titanium, with the remainder comprising iron and unavoidable impurities. The amounts of silicon and manganese are minimized, the amount of segregation and the like is reduced, and strength variation due to inter-coil position differences is decreased. The structure of the steel sheet is such that the area fraction of a ferrite phase is at least 95%, the mean diameter of ferrite crystal grains is at least 1 [mu]m, and TiC with a mean grain diameter of at most 7 nm is dispersed inside the ferrite crystal grains. This results in a high-strength hot-rolled steel sheet that maintains a yield strength of at least 530 MPa.

Description

technical field [0001] The present invention relates to a high-strength thin steel plate suitable for use as structural materials such as parts of conveying machinery such as automobiles, construction, etc., having a yield strength of 530 MPa or more and excellent stretch flangeability, and a method for manufacturing the same. Suppression of changes in mechanical properties within the material). The "steel plate" referred to here also includes steel strips. Background technique [0002] In recent years, in the automobile industry, from the viewpoint of protecting the global environment, the reduction of carbon dioxide gas CO 2 To improve the fuel efficiency of automobiles has always been an important issue. In order to improve the fuel efficiency of automobiles, it is effective to reduce the weight of the automobile body, but it is necessary to reduce the weight of the automobile body while maintaining the strength of the automobile body. By increasing the strength of the...

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Problems solved by technology

However, the technique described in Patent Document 2 has the following problems: the texture of the steel sheet cannot be stably obtained in the longitudinal direction and the width direction of the coil, and since martensite and residual Austenite, so the stability of strength is significantly reduced, and it is difficult to obtain stable and accurate formability

Furthermore, in the technology described in Patent Document 7, the distribution control of the clusters for each crystal grain promotes the variation of the strength of each crystal grain, and there is a problem that a stable material in the coil cannot be obtained.

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